1. Field of the Invention
The present invention relates to a method of heat processing a product, especially but not exclusively foodstuff, and relates particularly but not exclusively to the baking of bread.
The term "heat processing" is intended to cover, inter alia, the cooking of foodstuffs, including for example, the delicate vacuum cooking of meat, fish, vegetables or fruits, the pressure cooking of foodstuffs, and baking of farinaceous products such as bread.
With regard to bread, the time required to bake a loaf can be considerably shortened by baking a loaf's interior by microwaves and rapidly browning the exterior with conventional thermal heating, at temperatures higher than those usual for baking.
2. Discussion of the Prior Art
In industrialized countries it is common to cool bread to 21.degree. C. in order to slice it. Conventional cooling, achieved by conduction of heat from the interior of the loaf to its surface, demands a long period of time. From there heat is conducted to the surrounding air, which heats and leaves the loaf surface by convection currents. Forced air convection is frequently used to accelerate heat removal. The heat transfer is an unsteady state process, one major problem being the low heat conductivity of bread. Increasing the velocity of the surrounding cooling air does not significantly increase the cooling rate of the bread. Using such method a standard 800 gram loaf of white bread generally requires two and a half hours to cool.
Vacuum cooling is a technique which has been used for some years and can be used to cool standard loaves of bread in around three and a half minutes. Both microwave heating and vacuum cooling dehydrate the product to a greater degree than using the normal techniques of conventional thermal heating and cooling thereby increasing the weight loss normally resulting from these normal techniques. This creates an economic disadvantage for low profit margin product sold by weight such, for example, as bread. The advantages of microwave-assisted baking followed by vacuum cooling of bread are extremely high speed, the option of using soft wheat flour only (that cannot usually be used alone to bake bread) and a product with a very greatly reduced microbial contamination free of secondary contamination from cooling air, due to the airless vacuum cooling.